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About the introduction of EJB technology in JAVA language, Enterprise JavaBean (EJB) 1.1 specification defines the architecture of developing and deploying server-side software components based on transactional, distributed object applications. Enterprise organizations can build their own components or purchase components from third-party vendors. These server-side components, called Enterprise Bean, are distributed objects that reside in Enterprise JavaBean containers and provide remote services to clients distributed across the network.

An introduction to EJB Technology

The Enterprise JavaBean (EJB) 1.1 specification defines the architecture for developing and deploying server-side software components based on transactional, distributed object applications. Enterprise organizations can build their own components or purchase components from third-party vendors. These server-side components, called Enterprise Bean, are distributed objects that reside in Enterprise JavaBean containers and provide remote services to clients distributed across the network.

Two-tier and three-tier environment

In a two-tier client / server environment, programmers write applications that are closely integrated with vendor-specific software. Typically, two-tier applications access database services or transaction services directly from the client. Sometimes this application is called a fat client because the application logic resides on the client, which makes the client large and complex. The following figure depicts this feature:

A three-tier client / server application uses an intermediate or middle tier, the application server, which operates between the client application and the back-end database. The middle layer stores the business logic of the system and coordinates the display of interaction between the client and the back-end database.

There are two motivations for using a three-tier architecture on a two-tier model:

Improved scalability, availability, and performance

Improved business system flexibility and scalability

Because the two-tier system takes advantage of the processing power of clients, it has good performance, but many clients have bottlenecks on the characteristics of a single back-end resource, such as a database, as the number of clients increases gradually. this suppresses scalability, availability, and performance. The three-tier system tries to eliminate this bottleneck by managing back-end resources more effectively. This can be achieved by using resource management technologies, such as pooling and clustering middle-tier servers.

Pooling allows many clients to share inadequate resources, such as database connections, which reduces the workload on the back-end server and makes the three-tier system more efficient. Because multiple servers and resources can support failure recovery and balance the load of the growing number of clients, clustering can make three-tier systems more available and scalable.

A three-tier system is more flexible and scalable than the corresponding two-tier system because business logic and services, such as security and transactions, reside in the middle tier and are basically independent of the client application.

If the three-tier system is implemented correctly, in the case of Enterprise JavaBean, the service is automatically applied to the client request, so the service is invisible. Because the service is not visible to the client, changes to the service are not visible either. If the three-tier system is implemented correctly, changes and enhancements to business logic in the middle tier can also be hidden from client applications.

In addition, if the client and middleware components are implemented in the Java programming language, they are most likely portable. Class files that implement clients and application servers can be easily relocated to the current most appropriate host.

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